The present invention relates to the art of document reproduction. It finds particular application in conjunction with printer reproduction of color image signals having both pictorial and graphical elements.
The following patents are specifically incorporated by reference: U.S. Pat. No. 5,734,802 to Maltz et al. for its teaching of a blended look-up table for printing images with both pictorial and graphical elements; U.S. Pat. No. 5,483,360 to Rolleston et al. for teaching a color printer calibration with blended look-up tables; U.S. Pat. No. 5,471,324 to R. Rolleston for teaching a color printer calibration with improved color mapping linearity; U.S. Pat. No. 5,699,491, to R. Barzel for its teaching of a printer driver having gamut mapped colors.
The generation of color documents can be thought of as a two step process. In the first step, image signals (such as Red, Green and Blue in a RGB color space) representative of the image are produced by a scanner, work station or other image generating device. Thereafter, a printer, copier or other output device receives the image signals, converts them to printer or output signals (such as Cyan, Magenta, Yellow, or Black in a CMYK color space), and generates a reproduction. One problem with color reproduction is that devices have different characteristics. For example, print capabilities and colorants for printers are uniquely defined. As a result, a select image signal, when converted into a printer signal, may produce differing colors when printed by separate printers. To overcome this problem each printer is provided with a unique look-up table (LUT) for converting image signals into proper printer signals.
Each printer can print a limited range of colors (xe2x80x9cgamutxe2x80x9d) as faithful reproductions of the intended color. Typically, colors for scanned images, for example pictorial scenes, as opposed to colors for images generated by the work station, correspond to a portion of the printer""s available color gamut. These colors can be faithfully transformed into printer signals by a LUT that has been determined to accomplish a calorimetric match with the color intended by the user. In other words, scanned image signals for most colors are within xe2x80x9cthe gamutxe2x80x9d of the printer.
However, certain image signals, such as those relating to the colors of computer generated graphics, are outside of the gamut of the printer and cannot be reproduced faithfully; typically, computer generated colors are more saturated. Computer-generated, saturated colors cannot be faithfully reproduced within the gamut of the printer, therefore reproduction of such colors requires special translation, via a specially determined LUT, of the image signals into printer signals before printing. Typical examples of such colors are work station generated line art, bar graphs, or text.
As noted, xe2x80x9cpictorialxe2x80x9d image signals such as from a scanner or xe2x80x9cgraphicalxe2x80x9d image signals such as from a work station must be translated into an output, e.g. printer format, prior to being printed. in known systems, the translation is achieved using either a pictorial LUT or a graphical LUT. Both LUTs are held in a printer color conversion memory and are used depending on the type of signal to be translated, i.e., whether it is a pictorial image signal or a graphical image signal. Pictorial LUTs translate scanned image signals. Graphical LUTs translate saturated graphical RGB image signals into the most fully saturated colors the printer is capable of making. Graphical LUTs cannot be used to translate pictorial image signals, and, conversely, pictorial LUTs cannot translate graphical image signals, because they would generate numerous objectionable artifacts or unacceptable color reproduction.
While the foregoing designs have achieved successful reproductions, some image signals may be xe2x80x9cmixedxe2x80x9d, containing both graphical and pictorial elements. Some of these image signals are outside the gamut of a printer and cannot be translated into printer signals using merely the pictorial LUT. Likewise, the graphical LUT does not provide proper translation of these images signals into printer signals since the graphical LUT is merely directed towards fully saturated image signals.
Prior art solutions to this problem involve generating a blended look-up table, or alternatively compressing or clipping mixed image signals that are outside the range of the printer gamut into image signals fully convertible by the pictorial LUT. The prior art techniques of generating a blended look-up table, however, fail to generate satisfactory weights which are used in the blended look-up table, and therefore, outputs of existing blending systems generate undesirable artifacts and contours.
It is therefore desirable to provide a new and improved technique for building or generating weights which are used when blending pictorial and graphical color transformation look-up tables. It is therefore further desirable that weight building is designed in such a manner that transitions from one look-up table to another are smooth and do not create objectionable artifacts.
According to one aspect of the present invention, a method is provided for building weights used in the generation of a blended look-up table (LUT) which translates an image signal having both pictorial and graphical elements, into an output signal.
Weights are obtained by determining a position of a given input point from an input color space for both an input device gamut and an output device gamut. When the input point is located in the intersection of predefined percentages of the input device gamut and the output device gamut, the weight for the input point is set to a first predefined value. If the input point is outside at least one of the input device gamut and the output device gamut, the weight of the input point is set to a second predefined value. When the input point is within the predefined percentage of the output device gamut and anywhere inside the input device gamut, the weight of the input point is calculated according to the position of the input point relative to the input device gamut.
A determination is then made as to whether the input point is (i) within the output device gamut, (ii) outside a predefined percentage of the output device gamut, and (iii) inside the input device gamut. Next a test is undertaken to determine if the input point is in a predefined percentage of the input device gamut, when the previous step has determined the input point is in (i), (ii), and (iii), as described.
The weight is calculated according to the position of the input point relative to the output device gamut, when the testing determined the input point is in the predefined percentage of the input device gamut. Alternatively, the weight is calculated according to the position of the input point relative to both the input device gamut and the output device gamut, when the testing step determines the input point is outside the predefined percentage of the input device gamut.